Sea-level rise, a problem exacerbated by greenhouse gas emissions, could disrupt the lives of more than 13 million people in the United States, three times the most current estimates, according to a study published Monday.

Rising seas, which already endanger coastal communities through tidal floods and storm surges, could rise three feet or more over the next century if emissions continue at a high level, threatening many shoreline communities. The study, published in Nature Climate Change, argues that most projections vastly underestimate the number of people at risk because they do not account for population growth.

For the study, the authors combined future population estimates with predicted sea-level rise, using data from the National Oceanic and Atmospheric Administration, to demonstrate that millions are at risk: 4.2 million if seas rise by three feet, or 13.1 million with a six-foot increase, a high-end estimate.

Mathew E. Hauer, one of the study’s authors and a doctoral student in geography at the University of Georgia, said, “We could see a huge-scale migration if we don’t deploy any protection against sea level rise.”

Recent studies have shown that the sea levels are rapidly increasing, probably at the fastest rate in 28 centuries, and its accompanying tidal flooding, increasingly frequent, is already causing headaches in low-lying places, especially in the South. Though sea levels have risen and fallen significantly in the past, scientists say they have been fairly constant for the last several thousand years.

Mr. Hauer and his co-authors also found a highly regional effect of sea-level rise. Of the projected population at risk, nearly 50 percent will be in Florida, and an additional 20 percent in other parts of the Southeastern United States. In 30 counties, more than 100,000 people would be at risk if the sea level rose by about six feet.

None of the 22 coastal states in the continental United States, as well as Washington, D.C., will be immune from the effects of sea-level rise, the authors predict. If the seas were to rise by about six feet by 2100, more than one million people in California, and almost as many in New York and New Jersey, would be affected, the study shows.

The researchers estimated that the cost of relocating the 13.1 million people displaced by sea-level rise would be $14 trillion, based on relocation estimates for residents of Alaskan coastal villages.

Mr. Hauer said the study could be useful on a local policy level as well.

Predictions for sea-level rise are often done on a very small scale, while population forecasting is often done at a county or state level, Mr. Hauer said. Using census data, Mr. Hauer and his co-authors grouped units similar to city blocks to develop their forecast, and assumed that housing development patterns would continue at the same rate.

Benjamin H. Strauss, an expert on sea-level rise at Climate Central, a climate change research organization, said he believed that the new study overstated the number of people at risk, though he agreed that most estimates were too conservative.

The continuation of “current development patterns through the rest of the century seems like an unlikely future,” Dr. Strauss said, “because as sea levels continue to rise and coastal problems become glaringly obvious, coastal development and real estate will have to change.”

Dr. Strauss also said this flooding model lacked some of the nuance shown in other studies on sea-level rise. Different parts of the country will see different levels of sea-level rise at certain moments in time, he said: Coastal Louisiana and the Chesapeake region will see faster rates of change since the land there is also sinking, for example.

Another study published Monday in Nature Climate Change also highlighted the need for modeling that takes into account how different landscapes will react to sea level rise. The study, conducted by several members of the United States Geological Survey and a researcher from the Goddard Institute at NASA, demonstrates that several ecosystems in the Northeast, such as marshes and beaches, may adapt to sea-level rise, possibly undergoing formal changes. Developed or urban areas are more likely to flood or become submerged because of their hardened shorelines.

Erika E. Lentz, the report’s lead author and a research geologist with the United States Geological Survey, also said most places would not see the degree of inundation predicted in Mr. Hauer’s study.

Mr. Hauer, a demographer, said he was not making any predictions about the likelihood of certain sea-level scenarios, but was instead focused on the effects on people.

All three scientists acknowledged the degree of uncertainty in any of these forecasts. “We don’t have anything to compare this to,” Mr. Hauer said. “We just don’t know how people are going to act.”